Cytogenetic studies of patients with genetically determined or influenced disorders are conducted. Constitutional and secondary chromosomal variations and their relationship to phenotypic abnormalities segregating within families or occurring sporadically are evaluated. Chromosomal findings are also correlated with genetic markers such as HLA and alpha-1-antitrypsin. Previous studies of the Fragile X chromosome and Alzheimer disease are being concluded, and analysis of variation in the Nucleolus Organizing Region in Down syndrome families is in progress. A limited number of patients with genetic syndromes such as Prader-Willi, familial aniridia, and fragile X were studied as part of our participation in the NIH genetics program. Methods utilized include routine peripheral blood and tissue culturing, analysis of chromosomal response to folate-thymidine deprivation (fragile site induction), in-situ silver staining for detection of ribosomal gene activity of nucleolus organizing regions, chromosomal banding with Giemsa-trypsin or fluorescent stains, and methotrexate synchronization for analysis of prometaphase banding patterns. Screening of proband's relatives is carried out to determine if carrier status of chromosomal variations is causally related to phenotypic abnormalities. Results of all studies are correlated with clinical information and utilized for purposes of genetic counseling (predicting risk for development or recurrence of disease within families).